1. Field of the Invention
This invention relates to temperature sensitive polymer compositions which are used in flexible temperature or heat sensors for use in electric heaters.
2. Description of the Prior Art
Temperature sensitive polymer materials are generally used in the form of a sheet, which is provided between a pair of wound electrodes to form a flexible, temperature sensor or thermosensitive heater. The polymer materials ordinarily used for these purposes are specific types of polyamide compositions whose electrostatic capacitance, resistance or impedance varies depending on the temperature. This characteristic variation is utilized for temperature sensing.
The temperature sensitive polymer materials should generally satisfy the following requirements.
(1) Good temperature detection sensitivity. PA1 (2) Rapid melting when heated to temperatures exceeding the melting point to serve as a fuse. PA1 (3) undergoing little change, in relation to time, of electric characteristics by the action of ion polarization when a DC electric field is applied. PA1 (4) Suffering little influence of humidity.
Known temperature sensitive polyamide compositions meet requirements (1) and (2) in most instances. As for requirement (3), a number of factors greatly influence on the variation of electric characteristics. For instance, aside from the molecular structure of stabilizer used in the composition, the types of additives and/or the molecular weight of polyamide, the construction of a temperature sensitive device, i.e. a temperature sensor or a heat sensitive heater, and the application conditions of electric field show great influences. Requirement (4) is dependent largely on the characteristic properties of a temperature sensitive material and is not satisfied by the polyamide compositions presently used. This is because even nylon 11 and nylon 12, which have less moisture absorption than the other polyamide resins, are greatly influenced by humidity. A number of resin compositions which are improved in moisture absorption are known. For instance, Japanese Patent Publication Nos. 51-30958, 51-41237 and 53-117 disclose temperature sensitive compositions comprising mixtures of polyamide resins and phenolic compounds.
On the other hand, modified polyamide resins have been proposed for use as temperature sensitive materials, including polyamide resins containing N-alkyl-substituted amido units such as described in Japanese Patent Publication No. 57-59603 and Japanese Laid-open Patent Application No. 55-128203. Moreover, Japanese Laid-open Patent Application Nos. 57-206001 and 58-136624 describe polyester-amide resins, and Japanese Laid-open Patent Application Nos. 55-145756 and 55-145757 describe polyether-ester-amide resins.
Phenolic compounds which are mixed with polyamide resins must be high molecular weight compounds, which are low in heat loss, in order to satisfy the heat resistance requirement of the composition. High molecular weight phenolic compounds may be divided into two classes. One class includes compounds having high molecular weight groups, typical of which is a long chain alkyl group, and the other class includes polycondensates of phenolic compounds and aldehydes. The long chain alkyl group of the phenolic compounds should have not less than 11 carbon atoms in order to ensure a small heat loss. Phenolic compounds having a longer-chain alkyl group become less compatible with polyamide, with an attendant bleedout problem. On the other hand, aldehyde polycondensates are polymeric materials and tend to tangle with polyamide, so that they are unlikely to bleed out. However, phenol molecules are combined through methylene linkage, so that the portion including the phenolic hydroxyl group, which serves as a functional group, becomes so bulky that it is hindered from coordination with a highly crystalline amido group. This leads to the tendency toward a lowering of moisture resistance.
Polyamide resins containing N-alkyl-substituted amido units are disadvantageous in that when the degree of substitution of the N-alkyl group is increased, the melting point may lower to below 100.degree. C. Since the degree of substitution cannot thus be increased for use as temperature sensitive materials, it is difficult to suppress the variation of electric characteristics, such as electric resistance and/or impedance, caused by moisture absorption.
Resins having ester groups, such as polyesteramide resins or polyether-ester-amide resins, have a poor resistance to hydrolysis. In addition, in order to perform the fusing function as required for temperature sensitive polymer materials, the content of the amido groups in the resins is unable to be made so small and a great humidity resistance cannot be obtained.